FIG. 8 is a diagram conceptually illustrating a conventional hybrid electric-power supply system 90. For example, an electric vehicle, a refrigerated trailer, or the like, adopts the hybrid electric-power supply system 90 as shown in FIG. 8. The electric vehicle is disclosed in, for example, Japanese Patent Application Laid-Open No. 2000-152408 mentioned below.
In this hybrid electric-power supply system 90, when all switches 91 are conducting and all switches 92 are non-conducting, a three-phase power source 93 supplies three-phase AC power to a load 94 (an operation in such a connection relationship will be referred to as “plug-in operation”). When all the switches 91 are non-conducting and all the switches 92 are conducting, a power generator 95 supplies three-phase AC power generated by itself to the load 94 (an operation in such a connection relationship will be referred to as “normal operation”).